On Android 4 and above, Firefox doesn't have a Quit option in the menu. We'll show you how to add this with an add-on.

Tip: You can close Firefox or any other app from the app switch screen. Tap the app switching button
and then swipe an app off to the left or right to quit it. If you do not see the above button, long press the home button to bring up the app switch screen.

Add the Quit option to the menu

Tap on the menu button (either below the screen on some devices or at the top right corner of the browser)
, then Settings (you may need to tap More first)
, followed by Privacy.

Put a check mark next to Always clear when quitting and select at least one type of data to clear.

Add-on option: If you prefer not to clear your history every time you quit, you can install an add-on that adds the option to the browser. Note: The add-on developer is responsible for all updates and support.